The present invention relates to gamma radiation camera systems.
Gamma camera systems are employed in the medical field for imaging the radiation emitted by a radioactive tracer source injected into a patient. The tracers are designed to be absorbed by a particular part of the body and are made radioactive with a substance such as technetium 99. The radioactivity is short lived and usually of low energy, typically 100 keV. When the tracer has been absorbed the gamma ray camera can take a picture of the tissue in question, from outside the body, as an aid to diagnosis.
Such known camera systems usually incorporate a collimator which performs a function similar to a lens in an optical camera by selecting the rays which will form a useful image. The collimator is provided by a layer of gamma absorbing material, e.g. lead typically 1 cm thick, having a multiplicity of, e.g. 40,000, parallel holes therethrough. Thus, the field of view of the camera is limited to observation of gamma rays parallel to the holes.
As noted above, the energies of gamma rays to be detected in the medical field are relatively low, e.g. 100 keV or less. However, in other applications where it is required to detect gamma rays, e.g. in the nuclear industry to examine the radiation emitted from radioactively contaminated structures, the rays may be of much greater energy, e.g. in the range 500 to 1500 keV and a higher energy collimator with a wider, diverging field of view is required. As the capability of a collimator is extended to higher energies it is required to be thicker and hence heavier. The holes become fewer and of larger diameter. The resulting spatial resolution is poorer and the assembly is difficult to mount and control. For example a collimator for work up to 600 keV would weight typically 250 kg, not including any background shielding for the rear and sides of the detector. A collimator to image the 1332 keV radiation from Cobalt-60 would be so large and heavy that it would need its own separate mount.
The parallel hole collimator forms an image which has no optical equivalent. As noted above, an image will only be formed in a narrow field of view observing only rays travelling parallel to the collimator holes, whereas all conventional optical systems are divergent and the field of view can be varied.
There are two established ways of producing a diverging field of view in a gamma camera known in the prior art. The first is to provide a multihole collimator with diverging holes. This would have a fixed field of view but could be reversed to give a converging field. It would be so heavy, typically 250 kg, that it would only be suitable only for use at a fixed installation.
The second way is to provide a so-called "pin-hole" gamma collimator. This is analogous to the optical pin hole camera. This comprises a box around the detector or imaging plane, the box having a hole between the viewed scene and the detector. The problem is again one of shielding. The box of the camera must be massive to shield the imaging plane from unwanted radiation, thereby making the camera heavy and unwieldy. The box must be long enough to give a reasonable field of view, increasing further the weight of shielding involved. Typically, a shielding box weighing several tonnes may be required. The thick shielding also means that the hole must be of large diameter and specially shaped so that off-axis objects in the scene will still illuminate the image plane. The large hole results in poor spatial resolution.
The "pin-hole" method of imaging gamma rays has several advantages over the multihole collimator. The field of view of the pin-hole camera can be varied by altering the distance of the hole from the imaging plane. The camera can also be panned from side to side or up and down by moving the position of the hole parallel to the image plane rather than moving the whole camera. However, it is not easy to change or adjust the position of the hole. Furthermore, a pin-hole collimator is less sensitive than a multihole collimator as less of the radiation reaches the imaging plane through the single hole than through the many holes of the multihole collimator.